Disease, aging, trauma or chronic wear often lead to tissue or organ failure. In treating such failures, the goal of many clinical procedure is restoration of function. A patient often requires additional support, beyond the body's own means of healing, such as surgery or the implantation of a medical device. Such procedures are often needed to combat permanent disability and even death. The fields of biomaterials and tissue engineering are providing new options to gradually restore native tissue and organ function through the research and development of temporary scaffolds, matrices, and constructs (i.e., devices) that initially support a disabled tissue or organ, but eventually allow for the development and remodeling of the body's own biologically and mechanically functional tissue.
The responsibilities or design requirements of such a scaffold include: (i) the ability to provide immediate mechanical stabilization to the damaged or diseased tissue, (ii) support cell and tissue ingrowth into the device, (iii) communicate the mechanical environment of the body to the developing tissue; such is achieved through the proper mechanical and biological design of the device, (iv) degrade at such a rate that the ingrowing cells and tissue have sufficient time to remodel, thus creating new autologous function tissue that can survive the life of the patient. In certain instances, the device should mimic the correct three-dimensional structure (e.g., a bone scaffold) of the tissue it is attempting to support. In other instances, the device may serve as a temporary ligature (e.g., a flat mesh for hernia repair or a hemostat for bleeding) to a three-dimensional tissue (abdominal wall muscle in the case of hernia). Regardless of application, the present direction of the medical device field is the complete restoration of bodily function through the support of autologous tissue development.
Unfortunately, most biomaterials available today do not possess the mechanical integrity of high load demand applications (e.g., bone, ligaments, tendons, muscle) or the appropriate biological functionality; most biomaterials either degrade too rapidly (e.g., collagen, PLA, PGA, or related copolymers) or are non-degradable (e.g., polyesters, metal), where in either case, functional autologous tissue fails to develop and the patient suffers disability. In certain instances a biomaterial may misdirect tissue differentiation and development (e.g., spontaneous bone formation, tumors) because it lacks biocompatibility with surrounding cells and tissue. As well, a biomaterial that fails to degrade typically is associated with chronic inflammation, where such a response is actually detrimental to (i.e., weakens) surrounding tissue.
If properly designed, silk may offer new clinical options for the design of a new class of medical devices, scaffolds and matrices. Silk has been shown to have the highest strength of any natural fiber, and rivals the mechanical properties of synthetic high performance fibers. Silks are also stable at high physiological temperatures and in a wide range of pH, and are insoluble in most aqueous and organic solvents. Silk is a protein, rather than a synthetic polymer, and degradation products (e.g., peptides, amino acids) are biocompatible. Silk is non-mammalian derived and carries far less bioburden than other comparable natural biomaterials (e.g., bovine or porcine derived collagen).
Silk, as the term is generally known in the art, means a filamentous fiber product secreted by an organism such as a silkworm or spider. Silks produced from insects, namely (i) Bombyx mori silkworms, and (ii) the glands of spiders, typically Nephila clavipes, are the most often studied forms of the material; however, hundreds to thousands of natural variants of silk exist in nature. Fibroin is produced and secreted by a silkworm's two silk glands. As fibroin leaves the glands, it is coated with sericin, a glue-like substance. However, spider silk is valued (and differentiated from silkworm silk) as it is produced as a single filament lacking any immunogenic contaminates, such as sericin.
Unfortunately, spider silk can not be mass produced due to the inability to domesticate spiders; however, spider silk, as well as other silks can be cloned and recombinantly produced, but with extremely varying results. Often, these processes introduce bioburdens, are costly, cannot yield material in significant quantities, result in highly variable material properties, and are neither tightly controlled nor reproducible.
As a result, only silkworm silk has been used in biomedical applications for over 1,000 years. The Bombyx mori specie of silkworm produces a silk fiber (known as a “bave”) and uses the fiber to build its cocoon. The bave, as produced, includes two fibroin filaments or “broins”, which are surrounded with a coating of gum, known as sericin—the silk fibroin filament possesses significant mechanical integrity. When silk fibers are harvested for producing yarns or textiles, including sutures, a plurality of fibers can be aligned together, and the sericin is partially dissolved and then resolidified to create a larger silk fiber structure having more than two broins mutually embedded in a sericin coating.
As used herein, “fibroin” includes silkworm fibroin (i.e. from Bombyx mori) and fibroin-like fibers obtained from spiders (i.e. from Nephila clavipes). Alternatively, silk protein suitable for use in the present invention can be obtained from a solution containing a genetically engineered silk, such as from bacteria, yeast, mammalian cells, transgenic animals or transgenic plants. See, for example, WO 97/08315 and U.S. Pat. No. 5,245,012.
Silkworm silk fibers, traditionally available on the commercial market for textile and suture applications are often “degummed” and consist of multiple broins plied together to form a larger single multi-filament fiber. Degumming here refers to the loosening of the sericin coat surrounding the two broins through washing or extraction in hot soapy water. Such loosening allows for the plying of broins to create larger multifilament single fibers. However, complete extraction is often neither attained nor desired. Degummed silk often contains or is recoated with sericin and/or sericin impurities are introduced during plying in order to congeal the multifilament single fiber. The sericin coat protects the frail fibroin filaments (only ˜5 microns in diameter) from fraying during traditional textile applications where high-through-put processing is required. Therefore, degummed silk, unless explicitly stated as sericin-free, typically contain 10-26% (by weight) sericin (see Tables 1 & 2).
When typically referring to “silk” in the literature, it is inferred that the remarks are focused to the naturally-occurring and only available “silk” (i.e., sericin-coated fibroin fibers) which have been used for centuries in textiles and medicine. Medical grade silkworm silk is traditionally used in only two forms: (i) as virgin silk suture, where the sericin has not been removed, and (ii) the traditional more popular silk suture, or commonly referred to as black braided silk suture, where the sericin has been completely removed, but replaced with a wax or silicone coating to provide a barrier between the silk fibroin and the body tissue and cells. Presently, the only medical application for which silk is still used is in suture ligation, particularly because silk is still valued for it mechanical properties in surgery (e.g., knot strength and handlability).
Despite virgin silk's use as a suture material for thousands of years, the advent of new biomaterials (collagen, synthetics) have allowed for comparisons between materials and have identified problems with sericin. Silk, or more clearly defined as Bombyx mori silkworm silk, is non-biocompatible. Sericin is antigenic and elicits a strong immune, allergic or hyper-T-cell type (versus the normal mild “foreign body” response) response. Sericin may be removed (washed/extracted) from silk fibroin; however, removal of sericin from silk changes the ultrastructure of the fibroin fibers, exposing them, and results in loss of mechanical strength, leading to a fragile structure.
Extracted silk structures (i.e., yarns, matrices) are especially susceptible to fraying and mechanical failure during standard textile procedures due to the multifilament nature of the smaller diameter (˜5 um) fibroin filaments. The extracted fibroin's fragility is the reason that when using silk in the design and development of medical devices, following extraction, it is typically taught (Perez-Rigueiro, J. Appl. Polymer Science, 70, 2439-2447, 1998) that you must dissolve and reconstitute silk using standard methods (U.S. Pat. No. 5,252,285) to gain a workable biomaterial. The inability to handle extracted silk fibroin with present-day textile methods and machinery has prevented the use of non-dissolved sericin-free fibroin from being explored as a medical device.
Additional limitations of silk fibroin, whether extracted from silkworm silk, dissolved and reconstituted, or produced from spiders or insects other than silkworms include (i) the hydrophobic nature of silk, a direct result of the beta-sheet crystal conformation of the core fibroin protein which gives silk its strength, (ii) the lack of cell binding domains typically found in mammalian extracellular matrix proteins (e.g., the peptide sequence RGD), and (iii) silk fibroin's smooth surface. As a result, cells (e.g., macrophages, neutrophils) associated with an inflammatory and host tissue response are unable to recognize the silk fibroin as a material capable of degradation. These cells thus opt to encapsulate and wall off the foreign body (see FIG. 18A) thereby limiting (i) silk fibroin degradation, (ii) tissue ingrowth, and (iii) tissue remodeling. Thus, silk fibroin filaments frequently induce a strong foreign body response (FBR) that is associated with chronic inflammation, a peripheral granuloma and scar encapsulation (FIG. 18A).
In addition to the biological disadvantages of silk, the multifilament nature of silk (e.g., as sutures) as well as the small size of the fibroin filaments can lead to a tightly packed structure. As such, silk may degrade too rapidly. Proteases (enzymes) produced from the stimulated cells found within the peripheral encapsulation can penetrate the implanted structure (see FIG. 11A and FIG. 11B), but cells depositing new tissue (e.g., fibroblasts) which may reinforce the device (in this case a black braided suture) during normal tissue remodeling cannot. Therefore, the interior of non-treated or non-modified fibroin devices does not come in contact with the host foreign body response and tissue (led and produced by fibroblasts) and as a result, the capacity of the device to direct tissue remodeling is limited. Host cell and tissue growth is limited and degradation is not normally possible.
In the case of sutures, it is thought that these problems can be managed by treating fibroin sutures with cross-linking agents or by coating the sutures with wax, silicone or synthetic polymers, thereby shielding the material from the body. Coatings, such as sericin, wax or silicone, designed to add mechanical stability to the fibroin (combating its fragility while providing a barrier between the body and the fibroin), limits cell attachment, recognition and infiltration and tissue ingrowth and fibroin degradation. As a result, silk is traditionally thought of as a non-degradable material.
Classification as a non-degradable may be desirable when silk is intended for use as a traditional suture ligation device, i.e., cell and tissue ingrowth into the device are not desirable. Therefore, cell attachment and ingrowth (which lead to matrix degradation and active tissue remodeling) is traditionally prevented by both the biological nature of silk and the structure's mechanical design. In fact, a general belief that silk must be shielded from the immune system and the perception that silk is non-biodegradable have limited silk's use in surgery. Even in the field of sutures, silk has been displaced in most applications by synthetic materials, whether biodegradable or permanent.
Therefore, there exists a need to generate sericin-extracted silkworm fibroin fibers that are biocompatible, promote ingrowth of cells, and are biodegradable.